Free BJT Current Mirror Calculator
Compare standard two-transistor, Wilson, and Widlar current mirrors. Calculate reference current, base-current matching error, dynamic output impedance, and compliance range.
⚡ Mirror Topology & Supply Parameters
Beta Error: -1.64% • Early Effect Error: +4.32%
Thermal Runaway Warning: In discrete BJT current mirrors, the two transistors MUST be thermally coupled (placed in physical contact or using a matched dual transistor pair like BCM847). A 1°C temperature mismatch produces a 2mV V_BE shift, causing a ~8% current error!
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Principles of BJT Current Mirrors
A current mirror replicates a reference current through one active device into a second output branch, presenting an extremely high dynamic AC output impedance.
The Three Classic Architectures
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Basic 2-Transistor Mirror: Q1 has its collector tied to its base (diode connected). Q2 shares the same base-emitter voltage:
I_ref = (V_CC - V_BE) / R_refBase currents for both transistors are stolen directly from $I_{ref}$, creating an inherent error of $approx 2/eta$ (e.g. 2% error for $eta=100$).
I_out = I_ref / (1 + 2/β) × (1 + (V_CE2 - V_BE) / V_A)
R_out ≈ r_o = (V_A + V_CE2) / I_out -
Wilson 3-Transistor Mirror: Adds a third transistor (Q3) in feedback above Q1. This drives the base currents while cancelling base errors, reducing error to $approx 2/eta^2$ (e.g. 0.02%!) and boosting output resistance by:
R_out(Wilson) ≈ (β × r_o) / 2 [typically 5 to 20 Megohms!]
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Widlar Current Source: Introduces an emitter degeneration resistor ($R_E$) on Q2. This allows biasing microamp-level currents ($1 mu ext{A}$ to $50 mu ext{A}$) without requiring massive Megohm reference resistors:
ΔV_BE = V_T × ln(I_ref / I_out) = I_out × R_E2 (where V_T ≈ 25.9 mV)
Frequently Asked Questions
Why does the output current change when VCE2 increases?
The Early Effect (base-width modulation). As VCE2 rises, the collector-base depletion region widens, shortening the effective base width and increasing collector current. The transistor behaves as an ideal current source in parallel with an Early output resistance ro = (VA + VCE) / Iout.
Can I use discrete 2N3904 transistors for a current mirror?
Yes, but for accurate mirroring, glue their flat plastic faces together with thermal paste and wrap them in heat shrink tubing so their junction temperatures track identically.
What is the compliance voltage limit of a current mirror?
The compliance voltage is the voltage range across the mirror output where it remains in its active linear region. For a basic mirror, VCE2 must be > 0.3V (saturation limit). For a Wilson mirror, VCE must be > VBE + VCE(sat) ≈ 0.9V to 1.1V.